Method of treating silicon



March 29, 1960 J. R. LIGENZA 2,930,722

METHOD OF TREATING SILICON Filed Feb. 3. 1959 2 Sheets-Sheet 1 I E H 11?J2 WASH RINSE //v WASH m RINSE //v wAsl-l //v DE/ONIZED xmws BOILING //vH/VOa HF- HNO; WATER Ar 90%: WATER Ar IOOC. 1 v 1 RINSE //v RINSE[NI-I07 STEAM OXIDATION Ar Loam/1250 $22??? I I 7 500masoc. AND

WATER ROOM I SURFACE I 25-475 ATMOSPHERES DOP/NG 11 E sxpaszo IN J E iHF VAPOR 1 JZZZ' F/G 5 I 600 k FIG. 2

PRESSURE (ATMOsPHERES) TEMPERATURE (c INVENTOR J. R. L/GENZA BY ATTORNEYMarch 29, 1960 J. R. LIGENZA 2,930,722

METHOD OF TREATING SILICON Filed Feb. 3, 1959 2 Sheets-Sheet 2 FIG. 4

/NVENTOR By J. R. L/GENZA ATTORNEY United States Patent 2,930,722 7METHOD TREATING SILICON Joseph R. Ligenza, Westfield, N.J., assignor toBali Telephone Laboratories, Incorporated, New York, N.Y., a corporationof New York Application February a, 1959, Serial No. 790,848

' Claims. c1. 148-15) ICC should preferably-first be renderedhydrophilic, clean, and slightly oxidized.- This condition is achievedby methods comprising cleaning with an'organic solvent to remove waxesor other organic contaminants, rinsing in clean water, slightlyoxidizing the surface with an oxidizing agent, and rinsing again. Anumber of variations on this skeleton process may be devised. Thetreatment most advantageously used comprises etching the surface ofasilicon element in a mixture of hydrofluoric and nitric acids, rinsing,chemically cleaning by immersion irr a hydrocarbon solvent, rinsing,treating in hot nitric acid,'and rinsing. the element'once more beforesealing into autoclaves for steam oxidation; Other aspects of theinvention will become apparent from the accom panying drawings.

Fig. l-is a flow sheet of the preferred preparatory proeess and steamoxidation process herein described;

Fig. 2- is a'front elevation in section; greatly-enlarged, of adiodedevice comprising asingle'crystal body of silicon having" thereon asilicon oxide stabilizing film produced by the methods of thisinvention;

surfaces by a preparatory technique culminating in an f;

oxygen oxidation of the silicon at 900. centigrade to give a stableoxide thereon. The methods of the present invention teachapreferredpreparatory technique similar to that in they patentmentioned, followed by an oxidation of the clean silicon surfaces byclean steam under high pressures' and at lower temperatures than thoseheretofore used. The modified process has several significant advantagesover the process as formerly practiced.

v The use of steam under pressure as an oxidizing medium results in muchhigher rates of film growth than are possible by oxygen oxidation;Oxidation with oxygen of a silicon surface is rate limited by the speedof thermal dilfusion of silicon ionsffrom the bulk of the materialthrough the layers of oxide already formed on the surface. The morerapid growth rates observed when pressurized steam is employed arebelieved due to an attractive force exerted on silicon ions .in the bulkof the body by a high concentration of oxygen ions (from the Watermolecule) accumulated on the silicon surface. The attractiveelectrostatic force of these oxygen ions is believed responsible forpulling silicon ions to the surface. Thermal diffusion is no longer ratelimiting, and lower temperatures can be used' to accomplish theoxidation. The high growth rates and low temperatures possible in thepresent method permit formation of oxide films 5000 angstroms thick, orthicker, within periods which save considerable time over oxygenoxidation.

' Because lower temperatures are used for shorter periods 7 v of time informing an oxide coating by the new process herein described, there isless danger of shifting of junctions in devices having regions ofdifiering conduc- FigJB is a front elevation, partly in section, of asimple autoclave in which a number of: silicon semiconducting;elements'can simultaneously be oxidized with steam un- 'der highpressure; 1 1

Fig. 4 is a front elevation, partlyfi'insection of an autoclave systemfor the oxidation of silicon semiconducting elements in which'steam isgenerated at a source removed from the portions of the oxidation occurs;and j l r Fig. 5 is a pressuretemperature diagram defining thoseconditions of temperatureand pressure mostTsatisfactory for carrying outthe steam oxidations hereimdescribed. The flow diagram of Fig.shows-various steps involved in the preferred process of the. presentinvention; In step'I, a single crystal body of silicon is etched at roomtemperature in a mixture of nitric and hydrofiuoric acids, convenientlycomprising six parts by. volume of concentrated nitric acid. to one partby volumelof 48 percent hydrofluoric acid. Otheracid mixtures rang-.-ing in concentration from greater than 20 to 1 to less thanl to 1 can beused successfully, as known in the art. The etchant used is widely knownand is used in.

the art in all proportions. This step may be followedby an opticalquench in concentrated nitricacidtnot shown;

.on the flow sheet) which avoids the formation of surfacestains,possibly of silicon monoxide, which may other-,

wise form after the etching step.

Step II is a rinsing of the etched body in deionized water, which may becharacterized in having aconducg tivity of less than 0.05 micromho.

In step III, the washed body is treated by rinsing a.-

. continuously flowing distillate of a hydrocarbon solvent,-

tivity types. Lower temperatures and high growth rates also permit agreater variety of contacts tobe made to the silicon body beforetreatment without danger of loosening the contacts under the influenceofrprolonged heating. Also lower temperatures reduce or eliminatedifiusion of contact metals into the silicon substrate, which difiulsioncould otherwise lead to changes in electrical properties of the silicon.The avoidance of ele-. vated temperatures made possible by steamoxidation also minimizes the danger of devitrification of the amorphoussilica coatings produced. Devitrification can occur if silica films areheated to high temperatures in the presence of traces of certaininorganic materials.

It is to be understood that the steam oxidations here described havebeen relatively ineffectivej'alone in producing stable surfaces onsilicon without a preparatory cleaning of the surface. The surface to beoxidized of which benzene and xylene are exemplary. This;rinsing,continued for about fifteen minutes, aids in removing any organlcresidue on the surface of the semiconducting j body.

Traces of the solvent used for rinsing are removed by next rinsing thewashed element in boiling deionized water for about fifteen'minutes, asindicated forstep I-V of the flow chart in Fig. 1. if further cleaningof; the.

Patented/Mar. 29,, 1960* system in which silicon rinsing steps I and IIbefore I 3 tigrade, followed in step VII by a further rinse in a similarbath at room temperature. The steps to this stage have produced anexceptionally good silicon surface which is clean, hydrophillic, wet,slightly oxidized.

Depending on the characteristics finally desired in the silicon element,two alternative further treatments can be used. The treatment throughstep VII used on high purity, high resistivity, silicon produces asilicon surface which is lightly oxidized and almost perfectlyhydrophilic. If induced p-type conductivity is desired in the surfaceregions of such high purity silicon elements, the treated elements areimmediately subjected to a steam oxidation, as indicated in step IX. Toinhibit possible contamination or deterioration of the cleaned andpreparedsurfaces, this oxidizingstep should immediately follow step VII.The oxide induces a p-type conductivity in surfaceportions of highresistivity intrinsic silicon covered with the oxide.

However, if an n-type oxide induced conductivity region is desired insurface portions of a silicon element of high resistivity, additionalstep VIII may be practiced before the'oxidation of step IX. As indicatedin the flow chart, an n-type surface may be produced by exposing thesilicon surface to vapors of hydrogen fluoride, conveniently tohydrofluoric acid vapors, for a short time (for example, one to fifteenseconds) prior to oxi-- dation. This surface doping may be accomplishedwith various other vapors, for example chlorine, and various saltsolutions.

Another technique, not shown in Fig. I, for. producing an n-type surfaceregion comprises diffusing certain significant impurities, for examplegold or iron, into the silicon body before any surface treatment isbegun. During the oxidation, these impurities will be drawn from thebulk' of the material into the oxide film giving an ntypeconductivitysurface layer.

The oxidation of low resistivity silicon bodies, such as those priorlydoped for device uses, has no significant effect on the conductivitytype-of the underlying body. The effect of the oxide in inducing asurface conductivity type is slight, and becomessignificant only forintrinsic, high resistivity, silicon substrates.

Step IX, the oxidation by steam under pressure, is discussed in greaterdetail later in this specification.

Fig. 2 is a greatly enlarged sectional view of a silicon diode devicefabricated in part by the methods of the present invention. The elementcomprises a single crystal body of silicon having p-type region 20,n-type region 21, p-n junction 22 between them, and strongly n-type or n-type region 23. The p-n junction may be produced within the body bydiffusion techniques known to the art. For example, boron, which is adoping impurity for silicon, may be diffused into one side of an n-typewafer to produce a p layer and a p-n junction therebetween. Phosphorus,another significant impurity for silicon, may then be diffused into theother surface of the n-type wafer to give a three layered n+-n-pstructure like that in Fig. 2.

Metallic layers 24, of a material such as platinum, for example, maythenbe affixed to both sides of the wafer by means known to the art such asin a paste, or by evaporating, sputtering, or plating, to serve as lowresistance contacts for the device. Finally a central raised portion ormesa" is formed by known cutting or etching techniques, and the deviceis then processed as described in Fig. 1. Thin stabilizing protectiveoxide film 25 is thereby formed on the silicon surfaces. Film formationis particularly desirable on the mesa edges in the vicinity of-exposedp-n junction 22. As mentioned earlier, formation of the oxide has nosignificant doping efi'ect on the already doped silicon substrate.

Fig. 3 is a sectional view of a simple autoclave for carrying out thesteam oxidations of step 9 of Fig. 1. The outer casing of the autoclaveor bomb, comprising main portion of casing30 and threaded screw cap 31,is of a strong metal. The alloy Inconel X (73 percent nickel, 15 percentchromium, 7 percent iron, 2.5 percent titanium, 1 percent columbium orniobium, balance small mounts of aluminum, silicon, manganese andcarbon) has proved particularly successful as a material for the outercasing, but other strong materials are equally good. For example, analloy of percent platinum and 20 percent rhodium has been used withparticularly good results. Close fitting thin inner liner 32 of an inertmetal, conveniently gold, is within casing 30 to preclude possiblecontamination of the samples from the metals of casing 30. Cylindricaldisc 33 having a smaller disc 34 of inert metal, preferably gold, facedthereon, fits into screw cap"31. When cap 31 is tightly joined to casing30, disc 34 seals inner liner 32. Particularly if disc 34 is made of adeformable metal such as gold, a tight joint to liner 32 can be made. Inuse, the silicon elements are placed in the autoclaves with suflicientwater to produce a desired pressure at the temperature to be used inheating the bombs. To as sure that surface characteristics be uniform inany single element, it is important that thermal gradients, which favordifferential growth rates of the oxide films and variation in surfaceproperties, be avoided. For this reason, the bombs of Fig. 2advantageously are kept small in size, the longest dimension of thecasing being about two and one-half inches and that of the gold linerabout one inch. The water used in the oxidation step to produce steam ishigh purity deionized water as is employed in the silicon cleaningprocess. In addition, the autoclaves are cleaned prior to use by washingin nitric acid at centigrade, rinsing in hot deionized water, and thenrinsing again in deionized water at room temperature.

A convenient autoclave systemfor oxidizing a large number of samplessimultaneously is shown in Fig. 4. The system comprises double walledcabinet 41 with insulation 42, for example glass wool, between the wallsthereof. Strip heaters 43, comprising metal clad resistance heaters, aremounted in cabinet 41 as a convenient heat source for the cabinetinterior. Steam generating unit 44 is a thick walled autoclave havingdeionized water therein, and sealed to withstand high interiorpressures. Autoclave 44 shown is a commercial item, a product of theAmerican Instrument Company, Silver Spring, Maryland. Other similarapparatus could be used equally successfully, however. Tube 45 leadsthrough heated valve 46, also a commercial product, to tubular con-.tainer 47, in which the silicon elements to be oxidized are placed.Container 47 is ofa chemically'inert, structurally strong material.Though noble metals can be used, or a metal casing having a noble metallining, container 47 is conveniently made of silica with a wallthickness of about /8 inch. A pressure-tight seal of container 47 tocabinet 41 can be made by leading tube 45 to steel ring 48 having araised inner portion fitting into container 47. Second steel ring 49,fitting around container 47, presses lip 50 of container 47 against ring48. Gold washer 51, deformable under pressure, aids in forming agas-tight seal. Bolts 52 are provided on ring 48 so that ring 50 can beclamped tightly thereto, compressing lip 50 and washer 51. Furnace 53,conveniently comprises a coil of resistance wire, surrounds container 47to give uniform heating of all portions of container 47, minimizingthermal gradients.

The advantage of the autoclave system of Fig. 4 is the generation ofsteam at a pressure determined by the temperature of steam generatingunit 44. Steam at this pressure is fed through tube 45 and valve 46 tocontainer 47, where it is used at the temperature of furnace 53. Priorto use, all portions of the system contacting either the silicon to beoxidized or the deionized water used for the process are cleaned withhot nitric acid and,

, rinses of deionized water as earlier described.

Oxidation of silicon surfaces under conditions of too TABLE 1 MaximumTemperature (Degrees Centigrade) Pressures (Atmospheres) Althoughoxidation of silicon by steam takes place at even very low pressures, aminimum'steam pressure of 25 atmospheres, through the temperature rangeused, is

advantageously used so that thick. films can be formedf in feasiblyshort time periods. As shown in Fig. 5, steam pressures up to 475atmospherescan be used with good results in the lower temperature range.Temperatures between 500 centigrade and 850 centigrade are preferred forthe oxidations as shown in Fig. 5. Temperatures between 600 centigradeand 700 centigrade have given particularly good oxide coatings, and atemperature of 650 centigrade has been found to yield optimum results inmany instances. For the smaller temperature range mentioned above,maximum pressures, asshown in Fig. 5, range between about 465atmospheres and 250 atmospheres. At 650 centigrade the maximum pressureis 375 atmospheres. At this temperature and pressure, the growth rate ofoxide films is about 120 angstroms per minute, and a film 3000 angstromsthick can be grown in about 25 minutes. For'comparison, a film-ofcomparable thickness produced by prior art heatings in oxygen wouldrequire one hour at an elevated temperature of l250 centigrade. k

In the steam oxidation, increases in either temperature or pressurewithin the limits specified will increase the growth rate of oxide. I

At a given temperature and pressure, oxidation is continued until anoxide coating of desired thickness is .produced. Oxide fihns as thin as300 angstroms have been found useful in stabilizing surface properties,but thicker films up to 10,000 angstroms, and particularly between 5000angstroms and 10,000 angstroms, are usually preferred.

A specific example of the practice of the invention herein describedfollows below.

Example Silicon diode devices like those shown in Fig. 2 were made bydoping a sheet of n-type silicon by exposure to gaseous boron to form astructure having a p-n junction about 0.00l5 inch below thesemiconductor surface.

. One face of the wafer was then doped with phosphorus p-n junctions,the metal coated'portions 'of "the water: I

being protected by "wax. The wax was removedffby solvents, and thewafers rinsed in deionized water. The

wafers were then washed in xylene and rinsed again in The wafers werethen re-etched for deionized water. 7 five secondsin the 6:1 HNOHF'mixture, quenched in HNO and rinsed in deionized water. They werenext soaked for fifteen minutes in nitric acid at 100 centigrade, rinsedin deionized water for fifteen minutes, and

air'dried. p

The samples were oxidized in small bombs like those shown in Fig. 3,made of an 80 percent platinum-20percent rhodium alloy, with goldliners. The diodes were oxidized at 650 centigrade under a pressure of50 atmospheres for two hours, and had an oxide coatingv about 3000angstroms thick.

Table 2 presents the electrical characteristics of some of the diodestreated as described above.

TABLE 2 Reverse current at breakdown voltage less two volts Reversecurrent 10 volts (in amperes) 3 (IO- 4 (10 2.2 (IO- 3 (10") 2.5 (10-1.2.(10-

V 1.0 (10" r 4.6 (10' 3.0 (Ml- 2.6 (10" which method comprises washing abody of single crystal silicon in a mixture of nitric acid andhydrofluoric acid,

rinsing said body in deionized water, washing said body-1' 'in a flowinghot hydrocarbonv solvent, washing said body in boiling deionized water,immersing said body in hot nitric acid, rinsing said body in hotdeionzed water, rinsing said body in deionized water at room tempera-'-ture, and immediately thereafter oxidizing the surface of saidbodyfwithsteamat a temperature between. 500 centigrade'and 850 centigrade andbelow a maximum pressure of between 475 atmospheres at 500 centigradeand atmospheres at 850 centigrade fora period. sufiicient to produce anoxide layer having a thickness of at least 300 angstroms.

2. The method substantially as described in claim 1 which includes thestep of exposing said body of silicon to vapors of hydrofluoric acidjust prior to oxidizing the surface of said body with steam.

3; The method of fabricating a semiconductor device, which methodcomprises diffusing at least one significant impurity into a singlecrystal body of silicon, washing said body in a mixture of nitricv acidand hydrofluoric acid,

rinsing said body in deionized water, washing said body in a hothydrocarbon solvent, washing said body in hot deionized water, immersingsaid body in hot nitric acid, rinsing said body in hot deionized waterand then in deionized water at a lower temperature, and then oxidizingsaid body in steam at a temperature between 500 centigrade and 850centigrade and below a maximum pressure 300 angstroms.

4. The method of fabricating a semiconductor device, which methodcomprises preparing a clean hydrophilic surface on a body of singlecrystal silicon, and then oxidizing said body in steam at a temperaturebetween 500 centigrade and 850 centigrade and below a inaxlmum pressureof between 475 atmospheres at 500 centigrade, and 105fatmosphc'res'at'850" centigrade for a period sufficient to produce an oxide layerhaving a thickness of at least 300 angstroms.

5. The method of fabricating a semiconductor device, which methodcomprises washing a silicon body with an organic solvent, rinsing'inwater, slightly oxidizing the surface of said body, rinsing again inwater, and then oxidizing said body in steam at a temperature'between500 centigrade'and -850 Centigrade and below a maximum pressure ofbetween 475 atmospheres at 500 centigrade and 105 atmospheres at 850centigrade for a period sufiicient to produce an oxide layer having athickness of at least 300 angstroms.

References Cited in the file of this patent UNITED STATES PATENTS

3. THE METHOD OF FABRICATING A SEMICONDUCTOR DEVICE, WHICH METHODCOMPRISES DIFFUSING AT LEAST ONE SIGNIFICANT IMPURITY INTO A SINGLECRYSTAL BODY OF SILICON, WASHING SAID BODY IN A MIXTURE OF NITRIC ACIDAND HYDROFLUORIC ACID, RINSING SAID BODY IN DEIONIZED WATER, WASHINGSAID BODY IN A HOT HYDROCARBON SOLVENT, WASHING SAID BODY IN HOTDEIONIZED WATER, IMMERSING SAID BODY IN HOT NITRIC SAID, RINSING SAIDBODY IN HOT DEIONIZED WATER AND THEN IN DEIONIZED WATER AT A LOWERTEMPERATURE, AND THEN OXIDIZING SAID BODY IN STEAM AT A TEMPERATUREBETWEEN 500* CENTIGRADE OF 850* CENTIGRADE AND BELOW A MAXIMUM PRESSUREOF BETWEEN 475 ATMOSPHERES AT 500* CENTIGRADE AND 105 ATMOSPHERES AT850* CENTIGRADE FOR A PERIOD SUFFICIENT TO PRODUCE AN OXIDE LAYER HAVINGA THICKNESS OF AT LEAST 300 ANGSTROMS.